U.S. Pat. No. 6,498,685 (Ref. 1, incorporated herein by reference) disclosed an EUV lithography system comprising a xenon LPP (laser-produced plasma) illumination source, reflective illumination optics, microlens arrays, and microshutter SLM's (spatial light modulators), for achieving high-resolution, maskless printing. The illumination optics in the specific embodiment employed two Mo/Be (molybdenum/beryllium) multilayer mirrors for collimating the illumination. Mo/Be mirrors have a reflectance peak at 11.4 nm, which is well-matched to the xenon emission peak at 11 nm. Other prior-art EUV systems currently under commercial development (projection systems) use Mo/Si (molybdenum/silicon) mirrors, which are not as well-matched to the xenon source. (The Mo/Si reflectance peak is at 13.4 nm.) But Mo/Si mirrors are nevertheless preferred over Mo/Be due to the high toxicity of beryllium. The maskless system could also be designed to work with Mo/Si mirrors, but the efficiency of any system employing multilayer mirrors operating at near-normal incidence is significantly limited by the narrow bandpass of such mirrors (e.g., 0.27 nm FWHM for a Mo/Be mirror, and 0.56 nm FWHM for Mo/Si). As a result of this efficiency loss, EUV projection lithography systems are anticipated to have very high illumination power requirements, e.g., 400 W hemispherical source emittance in a 2% wavelength band at 13.5 nm. (This is for a throughput of 120 300-mm wafers per hour, Ref. 2.)
Prior-art EUV systems employing mask projection optics require near-normal-incidence mirrors, at least in the projection optics, in order to achieve high-resolution imaging of the mask onto the printing surface. But the collection optics need not necessarily use near-normal-incidence optics. For example, Cymer Corp. has been developing a system that uses a grazing-incidence collimator mirror (Refs. 3 and 4), and a similar system is disclosed in Ref. 5. However, the primary motivations for using a grazing-incidence mirror in this context are that its tolerance to source-generated debris is much better than Mo/Si mirrors and it is comparatively inexpensive. Improved optical efficiency is not a major consideration because the efficiency is fundamentally limited by the narrow bandpass of the projection optics mirrors.